Wind turbines provide a primary source of energy that can be converted into electricity and supplied to utility power grids, to which the generator is electrically connected. Conversion of wind energy to electrical energy is accomplished in a wind turbine by driving an electrical generator, commonly an AC induction motor. If the electrical power generated by a wind turbine is to be supplied to a utility power grid, then it is required to have a constant frequency that is synchronized to the utility line frequency, for example 50 Hz or 60 Hz. This can be accomplished by driving the generator at a constant rotational speed, which, unless a variable speed transmission is used, requires that the wind turbine rotates at a constant speed. Unfortunately, constant speed operation of a wind turbine limits its energy conversion efficiency due to variable wind conditions. Turbine rotor speed needs to be proportional to wind speed for optimal energy recovery.
Variable-speed wind turbines have been proposed as a way of increasing the energy conversion efficiencies of wind turbines. By varying the rotor speed according to varying wind conditions, improved energy recovery can be achieved over a range of wind speeds. Also importantly, the peak mechanical stresses caused by wind gusts can be reduced by allowing the wind turbine to speed up in response to wind gusts, thus limiting the torque reached on the generator of the wind turbine. The increased kinetic energy of the rotor caused by wind gusts serves as a short term energy storage medium to further improve energy conversion. Such operation, however, requires a responsive torque control system.
Although variable-speed wind turbines are advantageous from the perspective of increased energy conversion and reduced stresses, the electrical generation system is of necessity more complex than that of a constant-speed wind turbine. Since a generator is usually coupled to a variable-speed rotor through a fixed-ratio gear transmission, the electrical power produced by the generator will have a variable frequency. This requires a conversion from the variable frequency AC output by the generator to a constant frequency AC for supplying the utility power grid. The conversion can be accomplished either directly by a frequency converter or through an intermediate conversion to DC by a rectifier and reconversion to fixed-frequency AC by an inverter.
In a known realisation, the energy generated by the wind turbine generator is provided to a utility grid via a converter system that comprises at least an AC/DC converter, to a DC capacitor connected to a following grid-side DC/AC converter and ultimately to the utility grid, the signal so processed having suitable voltage and frequency levels. Such a solution, due to the fact that the energy generated from the wind turbine needs to pass through various expensive devices, makes the conversion of the generated wind energy unnecessarily expensive. Further, despite the various levels of signal processing, the generated signal, when ready to be fed into the utility grid, is not free of undesirable harmonic distortion components.
For example, U.S. Pat. No. 5,083,039 describes a power converter circuit comprising a number of active rectifiers and a number of active switching devices controlled by a control circuit. Due to the large number of components, the converter system is very expensive, and these costs have an impact on the cost of the energy fed into the utility grid. Further, the signal processed in this converter circuit may still not be free of undesirable harmonic distortion components.